Paleoarchean sulfur cycle and biogeochemical surface conditions on the early Earth, Barberton, South Africa

Abstract

This study presents the first multiple sulfur isotope dataset on sulfides from the ca. 3.5–3.2 Ga Onverwacht Group in the Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. In situ δ34SCDT and Δ33S values of pyrite (n=568) are reported from a wide range of hydrothermal, volcanic and sedimentary environments and are used to explore Mid-Archean biogeochemical sulfur cycling. Samples are from fresh drill core collected by the Barberton Scientific Drilling Project that intercepted cherts, metabasalts and sheared ultramafics of the ∼3.3–3.35 Ga Kromberg Formation; the sedimentary units of the ∼3.432 Ga Noisy formation; and the unconformably underlying metabasaltic pillow lavas of the ∼3.472 Ga Hooggenoeg Formation.Pyrite in quartz-carbonate-veins in the lower diamictite of the Noisy sequence records the largest range and most negative δ34SCDT values so far reported from an Archean terrain (δ34SCDT=−55.04 to +27.46‰). The Noisy sediments also contain detrital and diagenetic pyrites with a significant variation in Δ33S of between −0.62 to +1.4‰ and δ34SCDT of between −7.00 and +12.6‰, interpreted to reflect tectonic exposure of these early sediments to atmospheric – shallow marine conditions. Early marine pyrites from the Kromberg Cherts also display strong positive Δ33S values up to +2.50‰ with narrow range in δ34SCDT values (−6.00 to +1.50‰), whereas hydrothermal veins in the basal ultramafic shear zones preserve magmatic values (∼0‰). This study reveals a potential proto-tectonic control on atmospheric, geodynamic and hydrothermal environments available for early sulfate reducing and/or methanogenic microbes in the Paleoarchean.No evidence for microbial sulfate reduction or disproportionation was identified in the Kromberg Cherts, despite previous morphological claims for microbial life. Highly variable and negative δ34SCDT values were found in the Noisy turbidites and Hooggenoeg pillow lava breccia supporting the presence of microbial sulfate reduction in early tectono-sedimentary basins and in the Paleoarchean sub-seafloor, respectively. In light of current controversies surrounding sulfur isotope studies in similar-aged rocks of the Pilbara Craton (West Australia), we argue that microbial elemental sulfur disproportionation was not a preferred metabolic pathway on the Paleoarchean earth.